I owe Duke Williams a big thanks for suggesting that I convert my "ported" vacuum advance system to "full time" manifold vacuum. My 67 427/435 coupe with 411 gears ran beautifully from Montreal to Boston. No overheating while idleing and 170 degrees while cruising at 3500 to 4000 RPM. With the Duke recommended vacuum can and the "full time" vacuum line connections Duke suggested, this 427 has never run better or cooler...with no other changes. I hope this message will help others with a "ported" vacuum system. THANKS DUKE.
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Re: 67 427/435....Cool Runnin....Thanks Duke.
Glad it worked out, Rick. NOW, the next time someone has an overheating SHP big block, you can explain exactly how to solve it.
BTW, the correct VAC to use is a NAPA Echlin VC1765 or VC1808 or equivalent, which provides 16 deg. @ 12". The OE L-72 VAC is okay to use as it meets this spec, but L-71s have a 15.5" OE VAC, so one of the above or equivalent MUST be installed on a L-71 when the vacuum advance is converted from ported to full time. I think L-78 is full time vacuum advance, but it has a 15.5" OE VAC and this needs to be changed to a 12-incher.
I'm off to bigger and better things - nearly finished the L-72/71 Special design - massaged heads, custom designed cam using two different OE mechanical lifter lobes specifically indexed to the head flow characteristics. Hey those big blocks are even easy to stroke and no one will ever know.
Let me know if you want to tear into it and make it a REAL beast.
Duke -
PS...
... and thanks for sending me the copy of the '67 AMA Specs. One can never have too much data!
DukeComment
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Re: 67 427/435....Cool Runnin....Thanks Duke.
Do you have a date to where this was posted? A quick search didn't turn it up for me.Comment
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4.25" stroker is the only way to go!!! *NM*
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I'm not so sure!
Under Taylor's "similar engine" theory, increasing stroke - all other things being equal - the longer stroke engine will produce about the same peak power as the shorter stroke engine at the same mean piston speed, and any engine simulation program or real world testing will validate the theory. Of course, the longer stroke engine will produce more average power across the range because of its greater average torque, but it won't be as rev happy.
And any decent engine system engineer knows that increasing stroke requires reoptimization of valve timing to get the most out of a longer stroke configuration, particularly delaying the inlet valve closing point, so things don't have to be "otherwise all equal".
The trouble with pushing true net power beyond 400 HP in a C2 is exhaust backpressure. A massaged OE appearance mechanical lifter SHP/FI 327 will produce a true 325-350 net HP with about 3-3.5" psi backpressure through the 2.5" exhaust system. At 425 net HP exhaust system backpressure is approaching 6 psi, and significantly more of any indicated power increases are consumed by pumping loss. Exhaust system backpressure has a double whammy bad effect. First, for every one psi of exhaust backpressure, BMEP is reduced by an equal amount because of the pumping work. Even worse, exhaust backpressure reduces volumetric efficiency, especially when combined with high valve overlap because exhaust pressure is significantly greater than inlet pressure.
Now you know why the LS7 has 3" pipes and 120 degrees separation angle on its modest duration lobes - barely more effective overlap than a vintage base 350 cam, and much less than any of the Small block SHP or any vintage big block cam. I don't think GM understood this in the sixties (and the aftermarket still doesn't get it, today) because they did most of their power development work to SAE Gross conditions on lab dynos - no exhaust system backpressure.
Another issue with longer stroke configurations is bottom end durability as you keep coaxing more power at ever higher mean piston speeds.
It depends on individual preference, but a rev happy engine often feels better than a relatively low revving engine, even if the torque monster makes a little more peak power, and there's a much lower limit to how much power a C2 can effectively put to the ground (compared to a C6) at legal road speeds, even with the best available current tires that will fit in the C2 fender wells.
DukeComment
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even with a 4.00 stroke the new 427 ZO-6 engine
is chipped at 7000 RPM. this was high years ago for a SBCComment
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Re: even with a 4.00 stroke the new 427 ZO-6 engin
The redline on the tach is 7000, but I don't think the limiter hits until 7100, which works out to a mean piston speed of 4733 FPM, which only a handful of exotic production DOHC multivalve engines achieve.
Nextel Cup and F1 engines are running about 5200 FPM, which appears to be a physical limit based on sonic velocity and the time it take pressure waves to travel from the cylinder to the inlet plenum.
A vintage 327 is running a mere 3800 FPM at 7000 and a vintage 427 is running a little under 4100 at 6500.
Pushrod technology has come a long way!!!
DukeComment
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Re: 67 427/435....Cool Runnin....Thanks Duke.
Rick, would like to know more about this full time manifold vac.system you put on your car. I cant find Dukes original post. I also have a hot running L-71. Thanks, RandyComment
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street vs racing use
For street driving, the engine with the best torque curve will be the champ. The closed chamber large port heads as used in the 435 HP engines have a port that is "too large" for a 427 in usual "street" driving. Other than swapping heads, the most straight forward fix is to improve the mixture velocity by increasing the engine size from 427 to 489 (7 to 8 liters).
I can tell you from my own experience that in spirited street driving, my oval port 454's would always get the jump on my rectangular port engines in first gear and then tend to hold it until pretty far down the quarter mile. Remember, the car lengths you lose in the time it takes to get into the powerband take time to regain.
20 years ago, John Lingenfelter was working with the 496 strokers and he found that up to about 6000 RPM, they ran stronger with the oval port heads. I have had similar results with my engines - just how much are you really revving your rectangular port 427 and what RPM are you launching it at (once again, street vs. racing driving).
I think most here will agree that the 390 HP engine is actually a nicer "street" engine than the 435. Yes, the 435 revs higher and "sounds" better, but look at the 1/4 mile times with stock tires and the 435 really wasn't a whole lot faster!
MarkComment
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Re: street vs racing use
When you're dealing with an original numbers matching L-72, swapping heads is not an option. Depending on your perspective any mild engine can be more pleasant than a near beast. Consider the 300 HP 327 versus a 30-30 cam 327, and the 300 HP engine has plenty of power and rev potential without giving anything away with a lumpy idle, major loss of low end torque, loss of good driveability characteristics, or altering external appearance.
The early rectangular port BB heads are very restrictive on the exhaust side, but that can be mitigated with some port work, a larger valve, and a longer lobe on the exhaust side with the extra duration on the opening end. The inlet port has good flow for a SHP type torque curve, but even in OE trim a L-72 pulls 14" Hg. idling at 900 versus 10" on a 327 with a 30-30 cam, so the L-72 isn't that "radical" at all.
Oval port, rectangular port - massaged to your heart's content - it's all academic unless you are willing to increase exhaust system flow potential. That's the trouble with most of what you read in hot rod magazines. If they do dyno test development it's always on a lab dyno with headers, big exhaust pipes with an evacuation fan and no mufflers.
Then you put it in a chassis with the OE exhaust system (with or without headers) and it turns into a turd... happens all the time.
My prelimary system engineering investigation reveals that the L-72 with some head work and suitable valve timing will produce a nice SHP type torque curve and achieve close to 425 NET horsepower without any change in visual appearance of the engine or exhaust system or any fuel more exotic than unleaded premium.
Also of interest is that modeling the NET output of the L-72 as built by Tonawanda barely showed more top end power than my LT-1 cammed all original appearing SHP 327s. The L-72 heads definitely have some problems as built, but about 80 percent of the issues can be fixed including optimized valve timing for the massaged head flow, and it appears this will not hurt low end net torque while progressively raising the torque curve as revs rise for a substantial increase in top end power within the design speed of 6500 RPM.
DukeComment
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Re: 67 427/435....Cool Runnin....Thanks Duke.
Just go to the 2004 archives and type VC1765 in the search words box.
DukeComment
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we increased the exhaust port flow by installing
pressed in venturi type exhaust valve seats in cast iron heads. this was in race engines with open exhaust.Comment
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